The effect of dry wastewater sludge as sand replacement on concrete strengths

Abstract The ever-increasing population continues to have the ripple effect on the increase in sludge production. Dry sludge generally is heaped up on a landfill, which causes environmental pollution. Therefore, the study on the alternative use of dry sludge is of paramount importance. The dry wastewater sludge obtained from the Northwest treatment plant (NWTP) in Johannesburg, South Africa was used in this study as partial replacement of sand in concrete. Concrete were produced with 0%, 1%, 3% and 5% sand replacement with dry wastewater sludge. Compressive strength and splitting tensile strength tests were performed to study the effect of the dry sludge on concrete strengths. Samples for compressive strength test were prepared with fixed Water-cement (W/C) ratios of 0.67, 0.69 and 0.8 and cured for 3, 7, 28 and 90 days. However, samples for splitting tensile strength test were prepared with fixed Water-cement (W/C) ratios of 0.67 and 0.69 and cured for 28 and 90 days. The results showed a notable reduction of concrete strengths with an increase in sludge replacement. The compressive strength of 1% replacement at a W/C of 0.67 after 90 days curing recorded 16% reduction compared to the control sample, while 13% reduction was recorded for splitting tensile strength. The 90 days strength results showed that the sludge could be utilized (for non-structural purposes) as a partial sand replacement of concrete mixtures at a W/C of 0.67 with optimum percentage lower than 3%.